X-ray tubes are apparatuses mainly used in the medial field for medical diagnosis and treatment, and used in the industrial field for nondestructive testing, structural analysis, spectrum analysis, film exposure, etc., of materials. X-rays are harmful to human body, so effective protective measures are necessary during the use of X-rays.
X-ray tubes are vacuum electronic devices that generate X-rays by bombarding a metal target surface with high-speed electrons. A traditional X-ray tube generally adopts a thermionic emitting cathode, and heats materials such as tungsten (W) and lanthanum hexaboride (LaB6) to a sufficiently high temperature so that part of the electrons with larger kinetic energy overcome the surface potential barrier and escape from the material to form plasma around the material. When an external electric field is applied thereto, electron beams are emitted. However, such an X-ray tube is high in energy consumption and low in efficiency, and the conversion efficiency from electric energy to X-rays is less than 1%. Moreover, a considerably large amount of stray radiation are generated, and most of the electric energy is converted into thermal energy, so a large current is required during the use and the X-ray tube should be resistant to a high temperature.
Field emission needs no cathode heating, and a strong electric field is utilized to make the electrons near the surface of an object pass through the surface potential barrier to be emitted to the outside. The performance of field emission is dependent on the energy band structure of the material, work function, and the surface structure of the material. A field electron emission source features a large emission density, low energy consumption, a quick start-up, or the like. Field emission X-ray tubes all adopt carbon nano-tubes (CNTs) as cathodes (electron emission sources), and the CNTs have the following drawbacks due to growth techniques and structural limitation thereof, e.g., an insecure bonding between carbon tubes and bases, an insufficient voltage endurance capability with a highest voltage no more than 100 KV; the structure of the carbon tube is vulnerable when operating under a high voltage and tends to become incapable of emitting, meanwhile the vacuum degree within the tube is reduced, which makes damage to the ray tubes and shortens the service life; and poor uniformity in the direction of electron emission as a result of the disordered growth of the carbon tubes. In the prior art, no X-ray tube has adopted graphene as the cathode.